U.S. Pat. Nos. 4,812,247 and 4,812,248 of William Kohut describe a stable ball clay slurry that contained from about 50 to about 65 weight percent (by weight of dry ball clay) of silica. In the “comparative examples” discussed in such patents (see, e.g., comparative examples 10–89 of U.S. Pat. No. 4,812,427, at columns 39 et seq.), a discussion was presented of certain slurries made from clays with a silica content of 72 weight percent. Referring to columns 43 and 44 of U.S. Pat. No. 4,812,427, and in the examples 16, 17, 18, and 19 thereof, slurries made from clays containing 72 weight percent of silica were unstable, having a settling index of 0.11, 0.18, 0.16, and 0.18, respectively. By comparison, the stable ball clay slurry of U.S. Pat. No. 4,812,427 had a settling index of at least 0.9; but the maximum allowable silica content for such slurry was 65 percent.
The slurry described in the Kohut patents was not adapted to be used as a slip for casting. In order to make a casting slip, one must add kaolin, feldspar and flint to such slurry. The flint component is primarily comprised of crystalline silica, an ingredient which certain manufacturers have, for a variety of reasons, preferred not to utilize in their manufacturing processes.
It is an object of this invention to provide a clay slurry that can be used to make a casting slip with excellent properties and performance without the need to mix silica with such clay slurry or slip. It is another object of this invention to make such a slurry with a silica content of from about 68 to about 74 percent.
It is another object of this invention to provide a casting slip with a settling index of at least 0.9.
It is yet another object of this invention to provide a casting slip whose viscosity properties are relatively stable, varying by less than about 5 percent over a period of 3 days.
It is another object of this invention to provide a casting slip whose viscosity properties vary less than about 5 percent over a period of 10 days.
It is yet another object of this invention to provide a casting slip that can be used to make a cast green body that, when fired, will have less than 0.5 percent absorption.